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AIM 

6/17/21 

fighter will be more difficult to see on primary radar 
than a large commercial jet or military bomber. Here 
again, the use of transponder or ADS

B equipment is 

invaluable. In addition, all FAA ATC facilities 
display automatically reported altitude information 
to the controller from appropriately equipped aircraft. 

(f) 

At some locations within the ATC en route 

environment, secondary

radar

only (no primary 

radar) gap filler radar systems are used to give lower 
altitude radar coverage between two larger radar 
systems, each of which provides both primary and 
secondary radar coverage. ADS

B serves this same 

role, supplementing both primary and secondary 
radar. In those geographical areas served by 
secondary radar only or ADS

B, aircraft without 

either transponders or ADS

B equipment cannot be 

provided with radar service. Additionally, transpon-
der or ADS

B equipped aircraft cannot be provided 

with radar advisories concerning primary targets and 
ATC radar

derived weather. 

REFERENCE

 

Pilot/Controller Glossary Term

 Radar. 

(g) 

The controller’s ability to advise a pilot 

flying on instruments or in visual conditions of the 
aircraft’s proximity to another aircraft will be limited 
if the unknown aircraft is not observed on radar, if no 
flight plan information is available, or if the volume 
of traffic and workload prevent issuing traffic 
information. The controller’s first priority is given to 
establishing vertical, lateral, or longitudinal separa-
tion between aircraft flying IFR under the control of 
ATC. 

c. 

FAA radar units operate continuously at the 

locations shown in the Chart Supplement U.S., and 
their services are available to all pilots, both civil and 
military. Contact the associated FAA control tower or 
ARTCC on any frequency guarded for initial 
instructions, or in an emergency, any FAA facility for 
information on the nearest radar service. 

4

5

2.  Air Traffic Control Radar Beacon 

System (ATCRBS) 

a. 

The ATCRBS, sometimes referred to as 

secondary surveillance radar, consists of three main 
components: 

1.  Interrogator. 

Primary radar relies on a 

signal being transmitted from the radar antenna site 
and for this signal to be reflected or “bounced back” 

from an object (such as an aircraft). This reflected 
signal is then displayed as a “target” on the 
controller ’s radarscope. In the ATCRBS, the 
Interrogator, a ground based radar beacon transmit-
ter

receiver, scans in synchronism with the primary 

radar and transmits discrete radio signals which 
repetitiously request all transponders, on the mode 
being used, to reply. The replies received are then 
mixed with the primary returns and both are 
displayed on the same radarscope. 

2.  Transponder. 

This airborne radar beacon 

transmitter

receiver automatically receives the sig-

nals from the interrogator and selectively replies with 
a specific pulse group (code) only to those 
interrogations being received on the mode to which 
it is set. These replies are independent of, and much 
stronger than a primary radar return. 

3.  Radarscope. 

The radarscope used by the 

controller displays returns from both the primary 
radar system and the ATCRBS. These returns, called 
targets, are what the controller refers to in the control 
and separation of traffic. 

b. 

The job of identifying and maintaining 

identification of primary radar targets is a long and 
tedious task for the controller. Some of the 
advantages of ATCRBS over primary radar are: 

1. 

Reinforcement of radar targets. 

2. 

Rapid target identification. 

3. 

Unique display of selected codes. 

c. 

A part of the ATCRBS ground equipment is the 

decoder. This equipment enables a controller to 
assign discrete transponder codes to each aircraft 
under his/her control. Normally only one code will be 
assigned for the entire flight. Assignments are made 
by the ARTCC computer on the basis of the National 
Beacon Code Allocation Plan. The equipment is also 
designed to receive Mode C altitude information 
from the aircraft. 

NOTE

 

Refer to figures with explanatory legends for an illustration 
of the target symbology depicted on radar scopes in the 
NAS Stage A (en route), the ARTS III (terminal) Systems, 
and other nonautomated (broadband) radar systems. (See 
FIG 4

5

2 and FIG 4

5

3.) 

d. 

It should be emphasized that aircraft transpond-

ers greatly improve the effectiveness of radar 
systems. 

REFERENCE

 

AIM, Paragraph 4

1

20 , Transponder and ADS

B Out Operation 

4

5

Surveillance Systems